This invention relates to alternating current machines such as motors and generators.
A paper entitled xe2x80x9cNew permanent magnet excited synchronous machine with high efficiency at low speedsxe2x80x9d by Weh. H; Hoffmann. H and Landreth. J, Technische Universitat Braunschweig, Germany presented at the International Conference on Electrical Machines; Pisa 1988 describes a transverse flux arrangement which offered specific torque values much higher than the traditional radial flux configuration. The transverse flux machines disclosed are excited by permanent magnets which are mounted in alternating order upon the flux-guiding iron elements of the rotor. The magnets and the flux-guiding elements are arranged in a circumferential array around the periphery of the rotor, that circumferential array being arranged to rotate between pairs of axially spaced windings which are mounted in the stator. In order to reduce the influence of the stray flux, the rotor has to be made of non-magnetic material in the pole region except for the active parts of the magnetic circuit.
The transverse flux configuration combines a permanent magnet system with pole number 2p (i.e. wavenumber p) with an iron circuit which provides a special distribution of reluctance which also has a wavenumber p. The flux generated, being the product of magnetomotive force (mmf) and reluctance has a prominent zero wavenumber component which is employed as the working flux for energy transfer.
The disclosed transverse flux arrangement has two main drawbacks. Firstly, the flux component with wavenumber zero pulsates and so the generated torque also pulsates. This calls for at least two machine elements to be combined on the rotor shaft if a net constant torque is required, which is the case in most applications. Secondly, the construction of the permanent magnet and variable-reluctance elements of the machine is difficult since the flux follows paths which are inherently three-dimensional.
In a known form of alternating current machine which is known as a vernier reluctance machine, excitation is provided by a polyphase ac winding. Both the rotor and the stator have high wavenumber reluctance distributions, there being a difference in wavenumber equal to the wavenumber of the winding.
U.S. Pat. Nos. 5,386,161 and 5,532,531 respectively disclose three-phase permanent magnet stepper or brushless motors wherein the permanent magnets are part of the rotor. The brushless motor disclosed by U.S. Pat. No. 5,532,531 has a circumferential array of electromagnetic poles spaced by a uniform pitch and alternately magnetised. The permanent magnets on the rotor are also alternately magnetised and uniformly spaced with a pitch which is different from the pitch of the spacing of the stator poles.
DE-A-19643791 describes and illustrates a two phase servomotor based on an electronically commutated synchronous motor for relatively high output torque operation, comprising a rotor rotatable within a cylindrical stator with an annular air gap therebetween, the rotor having a circumferential array of teeth projecting towards the stator, and at least one pair of main poles per motor phase carrying working windings, the main pole pairs carrying permanent magnet excitation pole pairs which are mounted on the face of the respective main pole that faces the rotor and which have a pole pair pitch corresponding to the pitch of the rotor teeth, the excitation pole pairs being offset identically with the main poles from one motor phase to the other by                     o        ^            p        ⁢    H    =                    m        xc3x97                              o            ^                    z                    +                                                  o              ^                        ⁢                          :                                            2            ⁢            n                          ⁢                  xe2x80x83                ⁢        and        ⁢                  xe2x80x83                ⁢                              o            ^                    p                ⁢        H              =                  m        xc3x97                              o            ^                    z                    +                                                  o              ^                        ⁢                          :                                +          n                          2          ⁢          n                    
where
ÔpH is the main pole pitch
Ôz=tooth pitch
Ôp=excitation pole pair pitch
n=phase number
m=integer
EP-A-0373967 discloses an alternating current machine which includes two members of ferromagnetic material which form opposing boundaries of an air gap, one of which is movable relative to the other, one of the members having an armature winding arrangement and permanent magnets mounted on it, the magnets being arranged side by side in a line which follows the respective boundary of the air gap and being arranged to form a certain number of magnetic poles which are alternately north and south poles, the other member comprising a body portion and plurality of spaced projections which project from the body portion towards the one member in a direction which is transverse to the air gap, wherein each magnet is inserted into a respective groove which is formed in the surface of the one member that forms the respective boundary and which extends in a direction transverse to that boundary so that the surface of each magnet that faces the air gap is flush with the surface that forms that boundary.
One object of this invention is to provide novel configurations of alternating current machines which avoid the aforesaid main drawbacks of the disclosed transverse flux arrangement.
One subsidiary object of this invention is to avoid the need for electronic commutation.
Another subsidiary object of this invention is to provide an alternating current machine which offers high specific torque values.
According to one aspect of this invention there is provided an alternating current generator which is arranged to provide multipolar permanent magnet excitation in combination with multipolar reluctance distribution wherein different numbers of poles are used for the permanent magnet excitation on the one hand and for the reluctance distribution on the other hand so that the principal working flux has a finite waveform number which is equal to the difference in wavenumber between the mmf and permeance distributions.
According to another aspect of this invention, there is provided an alternating current motor which is arranged to provide multipolar permanent magnet excitation in combination with multipolar reluctance distribution wherein the motor has at least three phases whereby a traveling wave of mmf is produced substantially continuously and different numbers of poles are used for the permanent magnet excitation on the one hand and for the reluctance distribution on the other hand so that the principal working flux has a finite waveform number which is equal to the difference in wavenumber between the mmf and permeance distributions.
According to a further aspect of this invention there is provided an alternating current machine which is arranged to provide multipolar permanent magnet excitation in combination with multipolar reluctance distribution wherein different numbers of poles are used for the permanent magnet excitation on the one hand and for the reluctance distribution on the other hand so that the principal working flux has a finite waveform number which is equal to the difference in wavenumber between the mmf and permeance distributions, the machine including a cylindrical stator and a hollow cylindrical rotor which is mounted substantially coaxially with the cylindrical stator for rotation around the cylindrical stator with an annular air gap therebetween, the stator and the rotor being formed of ferromagnetic material, the stator having a winding arrangement and permanent magnets mounted on it, the magnets being arranged side by side in at least one circular line which follows the respective boundary of the air gap and being arranged to form a certain number of magnetic poles which are alternately north and south poles, the rotor being formed as a cylindrical body with a plurality of spaced projections which project radially from the body towards the stator, the number of projections being different from the magnetic waveform number that would be formed by a circular line of such alternately polarised magnets which were uniformly distributed along the whole of the respective boundary.
According to yet a further aspect of this invention there is provided an alternating current machine which is arranged to provide multipolar permanent magnet excitation in combination with multipolar reluctance distribution wherein different numbers of poles are used for the permanent magnet excitation on the one hand and for the reluctance distribution on the other hand so that the principal working flux has a finite waveform number which is equal to the difference in wavenumber between the mmf and permeance distributions, the machine including two members of ferromagnetic material which form opposing boundaries of an air gap and at least one of which is movable linearly relative to the other, one of the members having a winding arrangement and permanent magnets mounted on it, the magnets being arranged side by side in at least one line which follows the respective boundary of the air gap and being arranged to form a certain number of magnetic poles which are alternately north and south poles, the other member comprising a body portion and a plurality of spaced projections which project from the body portion towards said one member in a direction which is transverse to the air gap, the number of projections being different from the magnetic waveform number that would be formed by a line of such alternately polarised magnets which were uniformly distributed along the whole of the respective boundary.
Preferably the aforesaid alternating current generator or three phase motor includes two members of ferromagnetic material which form opposing boundaries of an air gap and at least one of which is movable relative to the other, one of the members having a winding arrangement and permanent magnets mounted on it, the magnets being arranged side by side in at least one line which follows the respective boundary of the air gap and being arranged to form a certain number of magnetic poles which are alternately north and south poles, the other member comprising a body portion and a plurality of spaced projection which project from the body portion towards said one member in a direction which is transverse to the air gap, the number of projections being different from the magnetic waveform number that would be formed by a line of such alternately polarised magnets which were uniformly distributed along the whole of the respective boundary.
The magnets may be mounted on the surface of said one member that faces said other member in which case the preferred ratio of thickness to width of each of the permanent magnets is substantially within the range of 0.2 to 0.4. Alternatively the magnets may be buried within said one member adjacent the air gap.
According to yet another aspect of this invention there is provided an alternating current machine which is arranged to provide multipolar permanent magnet excitation in combination with multipolar reluctance distribution and in which different numbers of poles are used for the permanent magnet excitation on the one hand and for the reluctance distribution on the other hand so that the principal working flux has a finite waveform number which is equal to the difference in wavenumber between the mmf and permeance distributions,the alternating current machine including two members of ferromagnetic material which form opposing boundaries of an air gap and at least one of which is movable relative to the other, one of the members having a winding arrangement and permanent magnets mounted on it, the magnets being arranged side by side in at least one line which follows the respective boundary of the air gap and being arranged to form a certain number of magnetic poles which are alternately north and south poles, the other member comprising a body portion and a plurality of spaced projections which project from the body portion towards said one member in a direction which is transverse to the air gap, the number of projections being different from the magnetic waveform number that would be formed by a line of such alternately polarised magnets which were uniformly distributed along the whole of the respective boundary, wherein the area of the surface of each magnet that affords the respective one of the magnetic poles is greater than the area that that magnet projects at the respective boundary of the air gap so that the flux density in the air gap associated with that magnet is greater than it would be if the surface of the magnet was flush with the respective boundary.
Preferably the surface of at least one of the magnets that affords the respective one of the magnetic poles is flat and oblique to the respective boundary.
Conveniently the magnets are flat and parallel sided. The magnets may be buried within said one member adjacent the air gap.
According to yet another aspect of this invention there is provided an alternating current machine which is arranged to provide multipolar permanent magnet excitation in combination with multipolar reluctance distribution wherein different numbers of poles are used for the permanent magnet excitation on the one hand and for the reluctance distribution on the other hand so that the principal working flux has a finite waveform number which is equal to the difference in wavenumber between the mmf and permeance distributions, the alternating current machine including two members of ferromagnetic material which form opposing boundaries of an air gap and at least one of which is movable relative to the other, one of the members having a winding arrangement and permanent magnets mounted on a surface of that one member which faces the other member, the magnets being arranged side by side in at least one line which follows the respective boundary of the air gap and being arranged to form a certain number of magnetic poles which are alternately north and south poles, the other member comprising a body portion and a plurality of spaced projections which project from the body portion towards said one member in a direction which is transverse to the air gap, the number of projections being different from the magnetic waveform number that would be formed by a line of such alternately polarised magnets which were uniformly distributed along the whole of the respective boundary, wherein the ratio of thickness to width of the permanent magnets is substantially within the range of 0.2 to 0.4.
The best choice for a compromise between the desire for high specific torque and high power factor is to use a ratio of thickness to width for each of the permanent magnets of about 0.2. On the other hand, where the desire for high specific torque is paramount, a ratio of thickness to width of each permanent magnet of 0.3 would be preferred.
In any aspect of this invention, the preferred number of projections which project from the body portion is half said certain number of magnetic poles plus or minus half the number of poles of the winding arrangement.
Each winding arrangement may comprise a respective projecting portion of said one member which projects towards said other member and on which a winding is wound. The magnets may be arranged in groups with a gap between each group which is mounted on a respective one of the projecting portions of said one member between the respective winding and said other member. Each of the number of projections may taper from the body towards the stator.
In an embodiment in which the magnets are buried within said one member, the magnets may be arranged in pairs, the magnets of each pair being of the same polarity, which is opposite to that of the juxtaposed pairs of magnets, and being arranged so that they converge radially inwardly, one on either side of a respective triangular pole piece, so that each triangular pole piece has the magnetic polarity of the convergent pair of magnets it is between. In this embodiment of the invention, each triangular pole piece is joined to the remainder of said one member by a respective bridge portion which passes between the nearer, radially inner ends of the magnets of the respective pair.
The lateral extent of each of the surface mounted magnets or the spacing between the outer ends of each pair of buried convergent magnets plus the thickness of the two magnets of that buried pair is preferably greater than the width of the outer end of each radial projection of said other member.
In an embodiment, the two members of ferromagnetic material may comprise a cylindrical stator and a cylindrical rotor, the cylindrical rotor being mounted substantially coaxially with the cylindrical stator for rotation relative thereto and with the air gap therebetween, the air gap being annular, the winding arrangement and the permanent magnets being mounted on the stator, the permanent magnets being spaced circumferentially one from another around the air gap, the rotor comprising a cylindrical body portion and said projections projecting radially from the cylindrical body portion towards the stator. Preferably the rotor has a laminated construction.
The stator may be a hollow cylinder with the rotor mounted for rotation within it. Alternatively, the rotor may be a hollow cylinder mounted for rotation around the stator.
In another embodiment, the two members may be arranged so that the movement of a first of them relative to the second is linear.
The one member may be stationary or it may be supported for linear movement relative to said other member.
Where relative movement between the two members is linear, the two members may be a first and a second of at least three such members, said second member separating said first member from a third member of ferromagnetic material, said second and third members forming opposing boundaries of another air gap and at least one of said second and third members being linearly movable relative to the other of said second and third members, said third member having a winding arrangement and permanent magnets mounted on it, these magnets being arranged side by side in at least one line which follows the boundary of the other air gap and being arranged to form a certain number of magnetic poles which are alternately north and south poles, there being a further plurality of spaced projections which project from the body portion of said second member towards said third member in a direction which is transverse to said other air gap, the number of these further projections being different from the magnetic waveform number that would be formed by a line of such alternately polarised magnets which are mounted on said third member and uniformly distributed along the whole of the boundary of said other air gap. Conveniently said first and third members are mirror images of one another as are the rows of projections formed on either side of said second member. The other or second member may be supported for linear movement relative to said one or said first and third members. In another arrangement, the winding arrangement and the polarities of the magnets on said third member may be the reverse of those on said first member so that the resultant paths of magnetic flux pass across said second member between said first and said third members.
An alternating current machine which embodies this invention may be a motor or it may be a generator. A thought process which a designer may pursue when designing a rotary machine which embodies this invention is described now by way of example with reference to FIG. 1 which illustrates interaction of mmf and permeance distribution in such a rotary machine to give distribution of working flux.
A notional stator having a circumferentially continuous periphery adjacent the rotor should be identified. It would have a number of alternately polarised magnets equi-spaced around that notional circumferentially continuous periphery which define a notional magnetic wavenumber (see FIG. 1). The rotor would be provided with a number of teeth which project towards the stator. That number of teeth would be any number which is different from the magnetic wave number. A number of permeance waveforms would be established equal in number to the number of teeth. There would be an interaction between the permeance waveforms and the notional magnetic waveforms which would create a number of flux waves which would be equal in number to the difference between the notional magnetic waveforms and the permeance waveforms.
The preferred form of armature winding arrangement comprises concentrated output coils. There may be one, two, three or more output phases and there would be at least one concentrated output coil for each phase. Alternatively the output arrangement may be in the form of a distributed winding.
In practice, in the case of a machine having two coils per phase, the stator also has to be provided with such a set of coils which are to interact with the flux waveforms and in which emf is to be induced. The stator would need to be provided with a number of cutouts, that number of cutouts being related to the number of coils. The choice of what material is to be removed from the notional stator is dependent upon the space that is required to accommodate the coils and other design criteria. The cutouts may be closed or semi-closed or open. If the cutouts are open or semi-closed, the periphery of the stator on which the magnets are mounted would be interrupted so that some of the magnets of the notional stator would not be fitted. There would be an even number of magnets on each of the remaining projecting portions of the stator between the cutouts in order to preserve magnetic balance. The design would be such that the flux distribution between the projecting portions of the stator remaining between the cutouts and on which the coils and magnets would be mounted would be the same as it would have been had the notional stator been used so that it is unaffected by the cutouts.
The overall vernier effect is not disturbed by the provision of open or semi-closed cutouts. However, in order to slightly increase the emf that is induced in the coils, it is beneficial to change the width of the stator projections and the spacing between the magnets thereon slightly so that the magnet spacing is such that the spacing of the north poles is the same as the tooth spacing. Hence that the magnetic wavelength is the same as the permeance wavelength locally as far as each stator projection is concerned although it is not so as far as the whole machine is concerned. This is because, at any one instant, only the magnets on one diametrically opposed pair of the projecting stator portions are aligned with the rotor teeth, the magnets on the other projecting stator portions being out of line with the teeth.